U.S. Pat. No. 2,219,646 (1940) assigned to Beecher and Mich have disclosed a process for the preparation of sodium silicate by dry fusion of silica with sodium hydroxide at temperature below the fusion or melting point of sodium hydroxide and about 2 to 4% soda ash is added to the reaction mixture. The source of silica could be silica sand, quartz, volcanic ash, silica flour etc. The drawback of the process is high temperature fusion of silica with sodium hydroxide and thus process becomes energy intensive and uneconomical.
U.S. Pat. No. 2,829,030 (1958) assigned to Habernickel wherein the process for producing alkali metal silicates having SiO2/Na2O mole ratio 2.74 to 3.16 is reported from the mixture of sand and alkali chlorides with about 2 to 4% other alkali metal salts like sodium carbonate, sodium bicarbonate, potassium carbonate and bicarbonate or mixture of them under the action of steam. The process involves the sintering step in which mixture of silica and alkali metal chloride is ground and then sintered at 780° C. to 840° C. and then cooled before steam treatment at 1150° C. to 1250° C. This process replaces conventionally used alkali carbonates and sulfates with cheaper alkali chlorides. However, the process is highly energy intensive as it is carried out at a high reaction temperature of 1150-1250° C. for 2-6 hours and involves pretreatment of sintering of the reaction mixture.
U.S. Pat. No. 2,988,423 (1961) assigned to McDaniel has described a process for the preparation of sodium silicate having SiO2/Na2O mole ratio in the range of 1.0 to 2.8 by reacting silica sand and aqueous solution of sodium carbonate at 155° C. to 217° C. under the pressure of 65 to 300 pound per square inch for about 20 hours or more. The drawbacks of the invention are the mole ratio of the sodium silicate prepared is comparatively low, high reaction pressure and temperature and very long reaction time make process uneconomical.
U.S. Pat. No. 3,984,526 (1976) assigned to Haase, et al. have disclosed a process for the preparation of alkali metal polysilicate solution having an SiO2:alkali metal oxide mole ratio of 2.5-5.5:1 and a silica content of 16-23% by weight which comprises contacting a tetraalkoxysilane of the formula Si(OR)4 wherein each R is independently a straight or branched-chain alkyl group of 1 to 3 carbon atoms with an aqueous alkali metal hydroxide solution at a temperature between room temperature and the boiling point of the reaction mixture and at a temperature up to 100° C., distilling over alcohol formed by the reaction of the silane with the alkali metal hydroxide. The drawback of the process is the source of silica is tetraalkoxysilane which makes process uneconomical.
U.S. Pat. No. 4,029,736 (1977) assigned to Melkonian, has disclosed a process for the preparation of water glass using perlite as a source of silica by treating the same with an alkaline solution having a concentration of 40-140 g/l taken in an amount which brings the ratio of the liquid and solid phases to (0.7-1.5): 1 and then separating by filtration the water glass, obtained in the process of heat treatment, from the residue formed. The drawback of the process is reaction of the perlite with alkali metal hydroxide at 130° C. to 150° C. under autogenous pressure.
U.S. Pat. No. 4,336,235 (1982) assigned to Deabriges Jean comprises a process for the manufacture of sodium silicate solution in a continuous manner from a silicon dioxide-containing material. This process includes continuous addition of a stream of said silicon dioxide-containing material and sodium hydroxide-sodium carbonate solution to a reaction zone at a temperature 225-245° C. and pressure 27-32 bar and for a time sufficient to form a sodium silicate solution, and recovering said sodium silicate solution from the reaction zone. In this process the stream of NaOH—Na2CO3 is pre-heated at temperature 250-280° C. in a nickel plated heat exchanger. This requires high cost equipment and operating condition shows that utility cost required is comparatively high which increases the production cost.
U.S. Pat. No. 4,520,001 (1985) assigned to Metzger, et al. have disclosed a process for the preparation of clear solution of alkali metal silicate having an SiO2/alkali metal oxide weight ratio of 2.5 or less which comprises passing an aqueous solution of an alkali metal hydroxide through a bed of crystallized silica having an average particle size of between about 0.1 mm and 2 mm formed in the bottom of a vertical tubular reactor without mechanical agitation, said silica and alkali metal hydroxide being fed from the top of the reactor, and recovering the resulting clear solution of alkali metal silicate from the bottom of the reactor. The reaction temperature was kept between 150° C. and 240° C. The drawback of the process is the SiO2/alkali metal oxide weight ratio is 2.5 or less. High SiO2/alkali ratio silicate can not be obtained and minimum 150° C. reaction temperature is required.
U.S. Pat. No. 4,676,953 (1987) assigned to Jeromin, et al. have described a process for continuous production of sodium silicate having mole ratio SiO2/Na2O 1-2.8:1 by fusing sand in aqueous sodium hydroxide solution at 150-300° C. and 5-40 bar pressure. The drawback of the process is high reaction temperature under high pressure which makes process uneconomical.
U.S. Pat. No. 4,770,866 (1988) assigned to Christophliemk, et al. have disclosed the process of producing sodium silicate by melting quartz sand and soda together at 1,400° C. to 1,500° C. in a suitable furnace with evolution of CO2. The melt that solidifies upon cooling in the form of a glass lump is then dissolved in water in another process step carried out under pressure at elevated temperature. The solution thus obtained is optionally filtered, depending upon the quality requirements. However, this process consumes very high amount of energy because of very high process temperature.
U.S. Pat. No. 5,000,933 (1991) assigned to Novotny, et al. has disclosed a process for the preparation of sodium silicate having high SiO2/Na2O molar ratio by reaction of a silicon dioxide source with aqueous sodium hydroxide solutions, or with aqueous sodium silicate solution using silicon dioxide source that contains sufficient fraction of cristobalite phase or by conditioning other crystalline forms or by conditioning other crystalline forms of silicon dioxide by heating at or above 1100° C., but below the melting point of silica, before the hydrothermal treatment. The reaction is carried out in a closed pressure reactor at temperatures of 150° C. to 300° C. and under saturated steam pressures corresponding to those temperatures. The drawback of this process is requirement of sufficient amount of cristobalite phase of SiO2 in the silica source and if not the silica source is required to be treated at very high temperature before hydrothermal reaction with sodium hydroxide solution. Another drawback of the process is the hydrothermal reaction is carried out in the range of 150° C. to 300° C. under autogenous pressure.
According to the above prior art, sodium silicate is prepared using quartz sand, cristobalite, perlite etc. and sodium carbonate or sodium hydroxide. Process known in the prior art for the preparation of sodium silicates are either energy intensive or operating conditions are such that utility cost is very high. Nothing is reported in published or patented literature wherein Kimberlite tailing has been used as a source of silica for the preparation of sodium silicate and other value added silica based products.